diff --git a/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.ComboBox.xaml b/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.ComboBox.xaml
index eb43db54f6..5746b3ddd4 100644
--- a/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.ComboBox.xaml
+++ b/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.ComboBox.xaml
@@ -534,7 +534,7 @@
                    PopupAnimation="Fade"
                    RelativeHorizontalOffset="-6"
                    SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
-                   Tag="{DynamicResource MaterialDesign.Brush.Background}"
+                   Tag="{DynamicResource MaterialDesign.Brush.ComboBox.DropDown.Background}"
                    UpVerticalOffset="15"
                    UseLayoutRounding="{TemplateBinding UseLayoutRounding}"
                    VerticalOffset="0">
@@ -557,7 +557,7 @@
         <ContentControl>
           <ScrollViewer MinHeight="1"
                   MaxHeight="{TemplateBinding MaxDropDownHeight}"
-                  Background="{Binding Background, ElementName=PART_Popup}">
+                  Background="Transparent">
             <ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Contained" />
           </ScrollViewer>
         </ContentControl>
@@ -796,8 +796,8 @@
           <Condition SourceName="PART_Popup" Property="wpf:ColorZoneAssist.Mode" Value="Standard" />
         </MultiTrigger.Conditions>
         <Setter Property="TextElement.Foreground" Value="{DynamicResource MaterialDesign.Brush.Foreground}" />
-        <Setter TargetName="PART_Popup" Property="Background" Value="{Binding Tag, RelativeSource={RelativeSource Self}, Converter={x:Static converters:RemoveAlphaBrushConverter.Instance}}" />
-        <Setter TargetName="PART_Popup" Property="Tag" Value="{DynamicResource MaterialDesign.Brush.Background}" />
+        <Setter TargetName="PART_Popup" Property="Background" Value="{Binding Tag, RelativeSource={RelativeSource Self}}" />
+        <Setter TargetName="PART_Popup" Property="Tag" Value="{DynamicResource MaterialDesign.Brush.ComboBox.DropDown.Background}" />
       </MultiTrigger>
       <MultiTrigger>
         <MultiTrigger.Conditions>
diff --git a/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Dark.xaml b/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Dark.xaml
index c0f104b5c7..ddc770bd20 100644
--- a/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Dark.xaml
+++ b/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Dark.xaml
@@ -28,6 +28,7 @@
   <colors:StaticResource x:Key="MaterialDesign.Brush.ComboBox.FilledBackground" ResourceKey="White100" />
   <colors:StaticResource x:Key="MaterialDesign.Brush.ComboBox.HoverBackground" ResourceKey="White100" />
   <colors:StaticResource x:Key="MaterialDesign.Brush.ComboBox.OutlineInactiveBorder" ResourceKey="White200" />
+  <colors:StaticResource x:Key="MaterialDesign.Brush.ComboBox.DropDown.Background" ResourceKey="Neutral100" />
   <colors:StaticResource x:Key="MaterialDesign.Brush.ComboBox.Popup.DarkBackground" ResourceKey="Neutral100" />
   <colors:StaticResource x:Key="MaterialDesign.Brush.ComboBox.Popup.DarkForeground" ResourceKey="Neutral900" />
   <colors:StaticResource x:Key="MaterialDesign.Brush.ComboBox.Popup.LightBackground" ResourceKey="Neutral900" />
diff --git a/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.DataGrid.ComboBox.xaml b/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.DataGrid.ComboBox.xaml
index 49840244c0..45e5533fe3 100644
--- a/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.DataGrid.ComboBox.xaml
+++ b/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.DataGrid.ComboBox.xaml
@@ -159,7 +159,7 @@
         <Grid MaxHeight="{TemplateBinding MaxDropDownHeight}" UseLayoutRounding="True">
           <AdornerDecorator CacheMode="{Binding RelativeSource={RelativeSource Self}, Path=(wpf:ShadowAssist.CacheMode)}">
             <Border x:Name="shadow"
-                    Background="{DynamicResource MaterialDesign.Brush.Background}"
+                    Background="{DynamicResource MaterialDesign.Brush.ComboBox.DropDown.Background}"
                     BorderThickness="1"
                     CornerRadius="2"
                     Effect="{DynamicResource MaterialDesignShadowDepth2}" />
@@ -167,7 +167,7 @@
           <Border x:Name="dropDownBorder"
                   MinWidth="{Binding ActualWidth, RelativeSource={RelativeSource AncestorType=DataGridCell}}"
                   Margin="{Binding ElementName=shadow, Path=Margin, Mode=OneWay}"
-                  Background="Transparent"
+                  Background="{DynamicResource MaterialDesign.Brush.ComboBox.DropDown.Background}"
                   BorderBrush="{DynamicResource MaterialDesign.Brush.DataGrid.ComboBoxHover}"
                   BorderThickness="1"
                   CornerRadius="2">
@@ -285,7 +285,7 @@
         <Grid MaxHeight="{TemplateBinding MaxDropDownHeight}" UseLayoutRounding="True">
           <AdornerDecorator CacheMode="{Binding RelativeSource={RelativeSource Self}, Path=(wpf:ShadowAssist.CacheMode)}">
             <Border x:Name="shadow"
-                    Background="{DynamicResource MaterialDesign.Brush.Background}"
+                    Background="{DynamicResource MaterialDesign.Brush.ComboBox.DropDown.Background}"
                     BorderThickness="1"
                     CornerRadius="2"
                     Effect="{DynamicResource MaterialDesignShadowDepth2}" />
@@ -293,7 +293,7 @@
           <Border x:Name="dropDownBorder"
                   MinWidth="{Binding ActualWidth, RelativeSource={RelativeSource AncestorType=DataGridCell}}"
                   Margin="{Binding ElementName=shadow, Path=Margin, Mode=OneWay}"
-                  Background="Transparent"
+                  Background="{DynamicResource MaterialDesign.Brush.ComboBox.DropDown.Background}"
                   BorderBrush="{DynamicResource MaterialDesign.Brush.DataGrid.ComboBoxHover}"
                   BorderThickness="1"
                   CornerRadius="2">
diff --git a/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Light.xaml b/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Light.xaml
index 68dac171db..8f00d024f1 100644
--- a/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Light.xaml
+++ b/src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Light.xaml
@@ -28,6 +28,7 @@
   <colors:StaticResource x:Key="MaterialDesign.Brush.ComboBox.FilledBackground" ResourceKey="Black50" />
   <colors:StaticResource x:Key="MaterialDesign.Brush.ComboBox.HoverBackground" ResourceKey="Black100" />
   <colors:StaticResource x:Key="MaterialDesign.Brush.ComboBox.OutlineInactiveBorder" ResourceKey="Black200" />
+  <colors:StaticResource x:Key="MaterialDesign.Brush.ComboBox.DropDown.Background" ResourceKey="Neutral900" />
   <colors:StaticResource x:Key="MaterialDesign.Brush.ComboBox.Popup.DarkBackground" ResourceKey="Neutral100" />
   <colors:StaticResource x:Key="MaterialDesign.Brush.ComboBox.Popup.DarkForeground" ResourceKey="Neutral900" />
   <colors:StaticResource x:Key="MaterialDesign.Brush.ComboBox.Popup.LightBackground" ResourceKey="Neutral900" />
diff --git a/src/MaterialDesignToolkit.ResourceGeneration/ThemeColors.json b/src/MaterialDesignToolkit.ResourceGeneration/ThemeColors.json
index 9f8172bd23..f3db724322 100644
--- a/src/MaterialDesignToolkit.ResourceGeneration/ThemeColors.json
+++ b/src/MaterialDesignToolkit.ResourceGeneration/ThemeColors.json
@@ -246,6 +246,15 @@
         },
         "alternateKeys": []
     },
+    {
+        "name": "MaterialDesign.Brush.ComboBox.DropDown.Background",
+        "themeValues": {
+            "light": "Neutral900",
+            "dark": "Neutral100"
+        },
+        "alternateKeys": [],
+        "obsoleteKeys": []
+    },
     {
         "name": "MaterialDesign.Brush.ComboBox.Popup.DarkBackground",
         "themeValues": {
diff --git a/tests/MaterialDesignThemes.UITests/WPF/ComboBoxes/ComboBoxTests.cs b/tests/MaterialDesignThemes.UITests/WPF/ComboBoxes/ComboBoxTests.cs
index e8e423c231..d74644be00 100644
--- a/tests/MaterialDesignThemes.UITests/WPF/ComboBoxes/ComboBoxTests.cs
+++ b/tests/MaterialDesignThemes.UITests/WPF/ComboBoxes/ComboBoxTests.cs
@@ -1,4 +1,5 @@
 using System.ComponentModel;
+using System.Windows.Media;
 using MaterialDesignThemes.UITests.WPF.TextBoxes;
 
 namespace MaterialDesignThemes.UITests.WPF.ComboBoxes;
@@ -269,4 +270,36 @@ public async Task ComboBox_BorderShouldDependOnAppliedStyle(string style, double
         Thickness thickness = await border.GetBorderThickness();
         await Assert.That(thickness).IsEqualTo(new Thickness(left, top, right, bottom));
     }
+
+    [Test]
+    [Description("Issue 3887")]
+    public async Task ComboBox_UsesDropDownBackgroundResource_WhenBackgroundIsNotSet()
+    {
+        await using var recorder = new TestRecorder(App);
+
+        var stackPanel = await LoadXaml<StackPanel>($$"""
+             <StackPanel>
+               <ComboBox Width="200">
+                 <ComboBox.Resources>
+                   <SolidColorBrush x:Key="MaterialDesign.Brush.ComboBox.DropDown.Background"
+                                    Color="#CC336699" />
+                 </ComboBox.Resources>
+                 <ComboBoxItem Content="Android" />
+                 <ComboBoxItem Content="iOS" />
+                 <ComboBoxItem Content="Linux" />
+               </ComboBox>
+             </StackPanel>
+             """);
+
+        var comboBox = await stackPanel.GetElement<ComboBox>();
+        await comboBox.LeftClick(Position.RightCenter);
+
+        var popup = await Wait.For(async () => await comboBox.GetElement<ComboBoxPopup>("PART_Popup"));
+        Color? popupBackground = await popup.GetBackgroundColor();
+
+        await Assert.That(popupBackground).IsNotNull();
+        await Assert.That(popupBackground).IsEqualTo((Color)ColorConverter.ConvertFromString("#CC336699"));
+
+        recorder.Success();
+    }
 }
diff --git a/tests/MaterialDesignThemes.UITests/WPF/Theme/ThemeTests.g.cs b/tests/MaterialDesignThemes.UITests/WPF/Theme/ThemeTests.g.cs
index b7063d9a20..498b8ffff3 100644
--- a/tests/MaterialDesignThemes.UITests/WPF/Theme/ThemeTests.g.cs
+++ b/tests/MaterialDesignThemes.UITests/WPF/Theme/ThemeTests.g.cs
@@ -53,6 +53,7 @@ private partial string GetXamlWrapPanel()
           <TextBlock Text="ComboBox.HoverBorder" Background="{StaticResource MaterialDesign.Brush.ComboBox.HoverBorder}" />
           <TextBlock Text="ComboBox.Border" Background="{StaticResource MaterialDesign.Brush.ComboBox.Border}" />
           <TextBlock Text="ComboBox.OutlineBorder" Background="{StaticResource MaterialDesign.Brush.ComboBox.OutlineBorder}" />
+          <TextBlock Text="ComboBox.DropDown.Background" Background="{StaticResource MaterialDesign.Brush.ComboBox.DropDown.Background}" />
           <TextBlock Text="ComboBox.Popup.DarkBackground" Background="{StaticResource MaterialDesign.Brush.ComboBox.Popup.DarkBackground}" />
           <TextBlock Text="ComboBox.Popup.DarkForeground" Background="{StaticResource MaterialDesign.Brush.ComboBox.Popup.DarkForeground}" />
           <TextBlock Text="ComboBox.Popup.LightBackground" Background="{StaticResource MaterialDesign.Brush.ComboBox.Popup.LightBackground}" />
@@ -345,6 +346,11 @@ private partial async Task AssertAllThemeBrushesSet(IVisualElement<WrapPanel> pa
             Color? textBlockBackground = await textBlock.GetBackgroundColor();
             await Assert.That(textBlockBackground).IsEqualTo(await GetResourceColor("MaterialDesign.Brush.ComboBox.OutlineBorder"));
         }
+        {
+            IVisualElement<TextBlock> textBlock = await panel.GetElement<TextBlock>("[Text=\"ComboBox.DropDown.Background\"]");
+            Color? textBlockBackground = await textBlock.GetBackgroundColor();
+            await Assert.That(textBlockBackground).IsEqualTo(await GetResourceColor("MaterialDesign.Brush.ComboBox.DropDown.Background"));
+        }
         {
             IVisualElement<TextBlock> textBlock = await panel.GetElement<TextBlock>("[Text=\"ComboBox.Popup.DarkBackground\"]");
             Color? textBlockBackground = await textBlock.GetBackgroundColor();
@@ -955,6 +961,7 @@ private static IEnumerable<string> GetBrushResourceNames()
         yield return "MaterialDesign.Brush.ComboBox.HoverBorder";
         yield return "MaterialDesign.Brush.ComboBox.Border";
         yield return "MaterialDesign.Brush.ComboBox.OutlineBorder";
+        yield return "MaterialDesign.Brush.ComboBox.DropDown.Background";
         yield return "MaterialDesign.Brush.ComboBox.Popup.DarkBackground";
         yield return "MaterialDesign.Brush.ComboBox.Popup.DarkForeground";
         yield return "MaterialDesign.Brush.ComboBox.Popup.LightBackground";